Golf is a sport which requires a choice of equipment of a quite special complexity. A professional champion, in order to strike a ball only 70 times over an 18 hole course, may use 15 different clubs whose characteristics will be adapted to each strike, these characteristics comprising the length of the shaft, the weight of the club and the shape of the head as well as its opening angle (called loft angle). The characteristics of the club will be selected here which depend essentially on the requirements of the players, that is to say, the length and flexibility of the shaft as well as the total weight of the club. These three essential parameters lead to "standardized" definitions, commonly admitted by all golfers and characterizing the "series" of clubs. By series of clubs is meant an assembly of about 12 clubs or so, more particularly 8 irons and 3 woods or 9 irons and 6 woods belonging all at one and the same time to one of the five classes of elasticity defined in accordance with the " Kenneth Smith" scale and designated respectively by L (ladies), A (flexible), R (medium), S (stiff), E (extra stiff) as well as to one or the other of the two classes of shaft lengths (men or women). It is noted here that there is no precise scientific definition of these classes of elasticity: where does medium stop to become a stiff?
It should be recalled that each of the 15 clubs of a series of 9 irons and 6 woods has a head which strikes the ball at a different angle (loft angle) allowing it to impart a more or less flat and more or less stabilized path depending on the spin given to the ball so as to take into account distances and obstacles which must be crossed. This loft angle is modified by the elasticity of the shaft. Thus, if a golf club manufacturer wishes to offer a fairly complete range of clubs, he must manufacture so as to take into account the five degrees of elasticity and the two degrees of length (men, women) which is ten series (5.times.2). The clubs of the same series have different characteristics but these characteristics have a certain homogeneity concerning particularly the characteristic which forms the "swing weight" which must remain substantially constant for the clubs of the same series. This "swing weight", a sort of moment of the striking force exerted on the ball, is calculated by multiplying the total weight of the club by the length of the distance which separates the center of gravity of the club from the theoretical point situated conventionally at 12 inches, that is to say 30.5 cm from the top of the shaft of the club (official scale swing-weight) or 15 inches (35.5 cm) from the top of the shaft (lorythmic swing weight). It is complex to take into account all the characteristic parameters of the clubs so as to manufacture a series of homogeneous clubs desired by the player (that is to say, a series of so called " consistent" clubs).
It is noted that it is also advantageous to lighten the shaft of a golf club. In fact, the energy E recovered by the ball at the end of the "swing" will be greater, that is to say, the ball will go farther, when the shaft is lighter. The energy given to the ball is such that E=1/2 MV.sup.2, M being the mass of the club and V the speed of the swing. It can be seen that for the same force, if the mass is low and the speed increases, the energy transmitted to the ball is in a proportion which increases with the square of this speed. It may then appear interesting to use light materials such as carbon fibers for manufacturing the golf clubs. However synthetic fiber shafts have other defects which have not up to present been easily overcome, in particular it is very difficult to obtain a series of "consistent" carbon fiber clubs as mentioned above.
It may observed that the physical characteristic which will take better into account all of the variables (weight, length, elasticity) is the frequency of vibration of the club for it integrates all of these parameters.
For a series of clubs to be homogeneous ("consistent") the vibration frequency of the clubs of the series must be constant or, if the player so desires, it must vary regularly from one club of the series to another.